The present invention relates generally to fiberoptic transmitters employing semiconductor laser diodes, and more particularly to techniques for operating and manufacturing the same.
Semiconductor laser diodes (often referred to below simply as lasers) are used in a variety of fiberoptic transmission systems for voice and data communications. As is well known, the output optical power of a forward-biased laser diode is a non-linear function of the drive current. For drive currents below a threshold current value I.sub.th, the output power is low, and the laser diode operates in what is known as the LED mode of operation.
For drive currents in excess of the threshold current value, the semiconductor laser diode operates in what is known as the lasing mode of operation. In this mode, the output power rises generally linearly with increases in current according to a parameter referred to as the laser diode's slope efficiency. The slope efficiency (S) of a laser diode is defined as the ratio of change in optical output power to change in laser drive current while the laser diode is in the lasing mode. It is thus common to provide a fixed bias current source that generates a current near I.sub.th and a modulation current source that provides additional drive current. The modulation current is modulated in accordance with a desired data pattern.
One measure of the operation is the extinction ratio (ER), which is defined as the ratio of the optical power in a logical 1 to the optical power in a logical 0. In general, the extinction ratio depends on the threshold current, the laser diode's slope efficiency, the bias current, and the modulation current.
The threshold current I.sub.th varies from diode to diode, and changes with operating temperature and age. This has resulted in difficulties in setting an appropriate operating point, or bias current, for the laser diode. One approach was to pick a suitable operating point for the particular laser diode at a particular operating temperature. However, because of the temperature characteristics of typical laser diodes, such prior art controllers would typically require a thermoelectric cooler (TEC) or other temperature control mechanism, leading to additional manufacturing and operating costs, as such cooling mechanisms are prone to failure in the field.
Published PCT Application No. WO 98/00893 [PCT/US97/112321] discloses a fiberoptic transmitter module that operates satisfactorily over a wide range of temperature without requiring a TEC. During calibration of a module, the module's laser diode is characterized over a defined operating temperature range. Characteristic data and/or curves defining the operational characteristics of the laser diode over the range of operating conditions (temperature, power supply) are stored in a non-volatile memory such as an EEPROM. During operation, an embedded microcontroller, together with analog-to-digital converters, digital-to-analog converters, and other associated circuitry, dynamically controls the operational parameters (e.g., modulation and bias current) based on the current operating conditions (temperature, power supply).
This design is extremely effective and powerful. However, the use of a microcontroller and the associated analog-to-digital converters and digital-to-analog converters tends to increase the size and cost of the module.